The long-term objective of this laboratory is to understand the molecular mechanisms and structural specializations by which the functions of muscle and connective tissue are integrated. In long muscles, notably those with parallel architecture such as the hamstrings, sartorius, adductors and abdominal muscles, the individual muscle fibers are shorter than the muscle, and terminate within the muscle by tapering to fine endings. In such muscles the motor nerve branches to innervate two or more muscle fibers in series. These facts make it necessary to postulate that such muscles are structurally comprised of "mechanical units" (as distinct from "motor units" or "muscle units"), where each "mechanical unit" consists of the specific muscle fibers and connective tissue that are necessarily involved in tension transmission from fascicle origin to insertion whenever a single motor unit is activated. This proposal is to determine how the muscle fibers of a mechanical unit are physically coupled to one another. Quantitative scanning and transmission electron microscopy, coupled with computer assisted image analysis, and quantitative light microscopy, will be used to test the hypothesis that the series-linked muscle fibers are non- contiguous, and that the tension sustained by partially activated mechanical units is transferred elastically form one active fiber to another through the intervening connective tissue and passive muscle fibers. The muscles will be analyzed for the presence of intercellular mechanical junctions and specialized fiber-to- connective tissue junctions. The morphological arrangement of the endomysium will be mapped in muscles fixed at different lengths in order to determine whether the collagen fibrils and bundles follow helical courses around the fibers, and, if so, whether the helical angle changes as the muscle fibers change length. The extent to which the collagen of the endomysium is shared by neighboring fibers will also be estimated. Morphological evidence in support of the hypothesis would suggest that parallel-fibered muscles should have an internal compliance that is an important aspect of their elastic and contractile properties. The results should significantly enhance our understanding of an important aspect of muscle biology which has heretofore been overlooked.